Precision Grinding of Hard and Brittle Materials

Pertanika J. Sci. & Techno\. Supplement 12(2): 113 - 125 (2004)ISSN: 0128-7680

© Universiti Putra Malaysia Press

Precision Grinding of Hard and Brittle Materials

V.C. Venkatesh, S. Izman, S. Sharif, T.T. Mon & M. KonnehDepartment of Manufacturing & Industrial Engineering

Faculty of Mechanical EngineeringUniversiti Teknologi Malaysia, UTM Skudai, Malaysia

E-mail: [email protected]

ABSTRAK

Dua teknik baru telah digunakan untuk mengkaji pembentukan jalur mod mulur semasamencanai intan (proses utama) ke atas germanium, silicon dan kaca untuk membantuproses sekunder penggilapan. Dalam teknik pertarna, permukaan aspherik telah dihasilkanatas Ge dan Si pada kelajuan konvensional (5000 pus per min). Dalam teknik keduapula, pencanaian pada kelajuan tinggi (100,000 pus per min) ke atas permukaan ratakaca dan Si telah dilakukan. Ketepatan bentuk, kemasan permukaan dan jalur modmulur dibincangkan dalam kertas keJja ini. Didapati bahawa roda intan ikatan resinmenghasilkan jalur mod mulur lebih banyak berbanding roda intan ikatan logam tetapisebaliknya roda yang terkemudian dapat menghasilkan ketepatan bentuk permukaanyang lebih baik. Permukaan' canaian berlutsinar diperoleh dengan lebih mudah atasPyrex berbanding kaca BK7, dengan itu telah dapat menjimatkan masa penggilapan.Jalur mulur kelihatan dengan banyaknya atas germanium berbanding silikon. Kedua-duateknik pencanaian baru ini telah dijalankan atas pusat pemesinan CNC.

ABSTRACf

Two novel techniques were used to study the formation of ductile mode streaks duringdiamond grinding (primary process) of germanium, silicon, and glass that aid thesecondary process of polishing. In the first technique asph~ric surfaces were generatedon Ge and Si at conventional speeds (5000 rpm). In the second technique high speedgrinding (100,000 rpm) of plano surfaces on glass and Si surfaces was carried out Formaccuracy, surface finish and ductile mode grinding streaks are discussed in this paper. Itwas found that resinoid diamond wheels gave more ductile streaks than metal-bondedwheels but better form accuracy was obtained with the latter. Transparent groundsurfaces were obtained more easily with Pyrex rather than with BK 7 glass, thusnecessitating very little time for polishing. Ductile streaks appeared in abundance ongermanium rather than silicon. Both the novel grinding techniques were used on CNCmachining centres.

Keywords: Grinding, diamond wheel, ductile streaks, polishing, Ge, Si and Glass, surfacef"mish and fonn

INTRODUCTION

Silicon, germanium and glass have become strategic materials that are widely used to'fabricate intricate components in microelectronics in the optical industry, and recentlyfor micro-electro-mechanical systems (Hung and Fu 2000). However, these materialspose particular problems in creating precision surfaces due to their extreme hardnessand brittle nature. Machining of these materials can result in surface alterationsincluding rough surface, cracks, subsurface damage and residual stresses, which later cancommonly be finished by lapping and/or polishing to meet high shape accuracy and lowsurface roughness requirements. In some instances, machining on these materials alsobecomes uneconomical due to short tool life and low material removal rate (Tonshoffet aL 1998).

V.C. Venkatesh, S. Izman, S. Sharif, T.T. Mon & M. Konneh

Recent advances in manufacturing technology make it possible for brittle materialsto be machined in ductile manner. According to fracture mechanics theory, brittlematerials can be removed by the action of plastic flow, as in the case of metal, leavingcrack-free surfaces when the removal process is performed at less than critical depth ofcut (Blake 1988; Blackley and Scattergood 1991). Critical depth of cut concept wasfurther investigated by other researchers such as Fang and Venkatesh (1998), Fang andChen (2000), Van et aL (2001) on various brittle materials. This concept has been foundto work well when machining hard and brittle materials using single point diamond toolon a rigid ultraprecision machine. It has been reported that fully ductile mode surfaceswith nanometric finish are possible when using this kind of machine but at the expenseof time and cost as the feed rate and depth of cut used are very fine. Bulk materialremoval is hardly possible using ultraprecision machines; therefore this process becomesless favourable for generating complex surfaces. As such, grinding process continues tobe the most viable method for generating complex surfaces like aspherics, spherics andtorics. Studies have shown that fully ductile mode machining was not possible withdiamond grinding due to difficulty to control grain depth of cut as the protrusion heightof diamond grits are randomly distributed allover the wheel surface. Instead, partialductile mode machining is feasible where minimum post-machining process is requiredif abundant ductile streaks are present. As explained by Konig and Sinhoff (1992), someof the flattened diamond grains that protrude at different heights on grinding wheelbecome critical depth of cut which causes plastic material flow during grinding, leavingthe ground surface with massive ductile streaks.

Traditional methods of generating aspheric surfaces on glass have been timeconsuming (Horne 1984). A novel technique was developed by Van Ligten and Venkateshwhich brought about heavy material removal without affecting surface finish and profile(Van Ligten and Venkatesh 1985; Van Ligten and Venkatesh 1986). The authors wereable to obtain surfaces, which satisfied ophthalmic conditions, but not those expectedof precision optics. This technique was extended to germanium and silicon using bothmetal-bonded and resinoid-bonded wheels.

The second technique involving plano surfaces was used on glass and silicon on a setup similar to high speed jig grinding. Resinoid and electroplated diamond wheels wereused to get transparency on glass and high quality surfaces on silicon. Surface finish,form accuracy, and smoothness were analysed on a Form Talysurf, while surface integritywas studied on Scanning Electron and Atomic Force Microscopes. SEM and AFMpictures show material removal both by ductile grinding and by fracture. These resultsare discussed in this paper.

METHOD

Aspheric Generation

To remove material quickly and end up with the desired surface, the contact areabetween the grinding tool and the work piece should be as large as possible. Since onlyspheres and toroids permit the condition of full area contact, partial area contact, or linecontact will be the best alternative. The use of a machine with a rotating tool suggeststhat the contact surface must be symmetrically rotational. In general, the shape of thework piece is not predictable, hence, the condition of the large contact area is put injeopardy. Thus, the method was chosen based on a long line contact between the tooland the work piece during the first step of rough grinding. During the subsequent stepsof lapping and polishing, the use of a flexible tool allows conformity between the workpiece and the tool, approaching the original condition of a contact area.

114 PertanikaJ. Sci. & Techno!' Supplement Vo!. 12 No.2, 2004


Two cup-shaped identical size diamond-grinding wheels with metallic (D20/30 MICL50M-l/4) and resinoid (SD240-RIOO B69-6mm) bonding were used. The profile ofthe grinding edge is circular in this case, but not restricted to this shape, thus forminga toroid. The important feature is that the grinding surface shape is axially symmetrical.It is now possible to program the path of this tool on the C C-machine such that it isin line (or arc) contact with the work piece as it cuts the desired shape on the glass.

To illustrate this, the grinding of a paraboloid is shown. The cup tool can be thoughtof as consisting of a collection of circles whose planes are perpendicular to the axis ofrotation of the tool. When the paraboloid is intersected by a plane, as shown in Fig. 1,the common line is an ellipse. When the task is to cut a concave paraboloid, the toolmust fit inside the paraboloid. Hence, the tool must have a diameter smaller than theshortest radius found on the ellipse of intersection of any plane intersecting theparaboloid. In the case of a paraboloid, the shortest radius of curvature on the eclipseof intersection is found when the plane contains the axis of symmetry of the paraboloid.

Any circle at the outer side of the tool can be contained in one of the planesintersecting the paraboloid. The angle that this plane makes with the axis of theparabolid can be adjusted such that the arc of the circle and that of the ellipse (see Fig.1b) at d, differs in sag height no more than a preset tolerance. This condition will seta certain common arc length over which the difference in sag does not exceed a certainvalue, say 0.5 micrometers. Now the work piece can be rotated around line 1 in Fig. 1a.After completion of one revolution, the surface left on the glass workpiece will be a zoneof the paraboloid, as indicated in Fig. 1a. Subsequently, the tool axis can be programmedto take a slightly different position relative to the axis of rotation of the work piece, aswell as relative to the apex P, of the paraboloid. The sequence is then repeated to forma neighbouring zone of the one indicated in Fig. 1a.

B

paraboloid

(b)

Fig. 1: A plane intersecting a paraboloid (a), in an ellipse (b)

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Two 5-axis vertical CNC machining centres (Fadal and Deckel) (Venkatesh 1990)were programmed to perform these sequences continuously, until the paraboloid iscompleted. In this manner, a zone is ground during one revolution. While in cuttingwith a point tool, in principle, one line of the paraboloid is cut. It is thus clear that thismethod of grinding with the nearest arc-eontact can complete the same surface fasterthan in the case of cutting with a point- tool. The thermal imaging materials used weremonocrystalline germanium and silicon. Both blanks were polished after grinding. Aspecial aluminium tool was developed. A felt cloth was glued to the spherical surface ofthis tool and a polishing paste of one micron alpha alumina was applied to it duringpolishing. The same set-up was used for polishing on the CNC machine.

Plano Grinding

The objective here is to grind plano (flat) surfaces, with the areas being quite small(lOxI2mm2). Very high quality Si as thin as Imm (wafer) and as thick as 10mm (lenses)and optical glass (BK7 and Pyrex) were used. An air-powered jig grinding unit (NSKPlanet 1500) was attached to the vertical spindle of a MAHO CNC Milling machine toprovide tool path movement in plano grinding. One advantage of using an air-drivenlow powered jig grinder is that it stalls when cutting forces exceed 10 Newton. Theoperating pressure of the Planet was set constant to 4 kgf/cm2 throughout the experiment.The uneven cut surface on BK7 Schott glass workpiece that produced after diamondsawing was flattened prior to plano grinding using a resinoid cup wheel (SDC230PI00BW4-6). For rough grinding (flattening) and fine grinding operations the samegrinding tool (grinding pin) was used for wafer applications and separately for the lensoperation to avoid heat accumulation in the former application. For both work pieces,resinoid-bonded mounted wheels (IAIW-5-6-1.5/DI26/ClOO) also known as grindingpins because of their size (4) 5mm) were used as grinding wheels.

Special fixtures were designed to hold different work piece geometry and at thesame time to provide rigidity during grinding. For both materials, rough and planogrinding were carried out in a single setting without removing the work piece from thefixture so that the flatness could be maintained. A close-loop coolant system completewith filtering unit was fabricated to ensure that only clean coolant was suppliedcontinuously to the work piece during grinding. Castrol Miracol 80 coolant with ratio1:50 was used. Fig. 2 shows this experimental set-up. Three different feed rates (5, 20 and35mm/min) and three depths of cut (5, 15 and 25 mm) were used to grind silicon andBK7 glass in plano grinding. However for Pyrex glass, an additional feed of 2.5mm/minand depth of cut of 10mm were used. The wheel was dressed with Al20~ bar (grade Nr.2)each time before starting a new grinding condition.

Surface roughness, form accuracy, and smoothness of all samples were analysed ona Form Talysurf and Surfcom 1800, while surface integrity was evaluated on ScanningElectron, Atomic Force and Optical Microscopes.

RESULTS AND DISCUSSION

Aspheric Surfaces on Ge and Si

Aspheric profile generation on germanium and silicon lenses is usually accomplishedusing the single-point diamond turning method, but it is being challenged by a noveltechnique utilizing a CNC 5- axis machining center instead of a dedicated machine. Thisnew method is characterized by zonal material removal using cup-shaped diamond grinding wheels. Determination of optimum grinding wheel bond type and grit size isof great importance in the success of this grinding method.

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Fig. 2: High speed jig grinding of silicon and glass

Two grinding wheel bond types, metal and resinoid, each with three different gritsizes, 10-30 mm, 40-60 mm and 50-70 mm were used in generating aspheric surfacesusing the grinding zone method. Surface texture and profile accuracy of each groundworkpiece were measured and analysed using the Rank-Taylor-Hobson Form Talysurf.Also, the Scanning Electron Microscope was used to determine whether the materialremoval mode was predominantly fracture or ductile.

The resinoid-bonded wheel was superior for both Ge and Si. It could be redressedand trued, and commercial sizes are available. The metal-bonded wheel was made to ourspecifications and had considerable run out (0.279 mm) which could not be corrected.Better surface roughness values were obtained with both wheels for Si (Table 1). Si,however, was more difficult to polish and a lighter pressure had to be applied to preventthe felt from coming off. Thus, for the same time interval, Ge had a much better surfacefinish (Table 1). Polishing improved the form accuracy with Ge, but not with Si (Table1). Both ductile and fracture modes of material removal were observed with both Si andGe. Fig. 3 shows some amount of ductile streaks on silicon and Fig. 4 a massive amount

TABLE 1Average roughness and form accuracy after grinding (G) and after polishing (P)

Ge SiType of

Grinding Wheel Ra Form Ra Form(mm) Accuracy (mm) (mm) Accuracy (mm)

Metal-bonded - G 0.722 4.16 0.583 6.03Metal-bonded - P 0.057 2.867 0.123 6.490Resinoid-bonded - G 0.706 1.845 0.123 1.898Resinoid-bonded - P 0.045 1.238 0.191 2.433

PertanikaJ. Sci. & Techno!. Supplement Vo!. 12 0.2,2004 117

Ye. Venkatesh, S. Izman, S. Sharif, T.T. Mon & M. Konneh

of ductile streak on Ce. Better surface roughness, form accuracy, and smoothness canbe obtained with a 5-axis C C jig grinder, and also by dressing the grinding wheel byductile mode as suggested by Miyashita (1988).

Plano Surfaces on Silicon and Glas~

Results indicate that the surface roughness of precision ground Si improves with lowerfeed except in the case of the finest depth of cut of 5mm where higher feed rateimproves the finish (Fig. 5). Ductile streaks also appear at higher feed rates (Fig. 6). Fig.7 is an AFM picture showing a mirror-polished surface of ground Si that had ductilestreaks.

Fig. 3: Ductile streaks obtained on Ge during aspheric generation

Fig. 4: Massive formation of ductile streaks on Ge during asphericgeneration with a resinoid-bonded diamond wheel


(a)

(b)


Surface Roughness vs Feed rate(Grtt 0126- Monocryslalllne 51)

0.16149

0.14 0.1170.126

0.12

E0.1

.2- 0.08-+-doc = Sum.. "'-doc = 15um

'" 0.06 -'-doc = 25um

0.04 0057

0.02

05 20 35

Feed rot. (mmlmln)

Fig. 5: Surface roughness obtained when silicon dies were precisionground at different levels oj depth oj cut and Jeeds

Fig. 6: S1'.M pictures oj ground Si. The surfaces consist oj(a) microJraetures and (b) grinding streaks

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1\TOMIC FORCE MICROSCOPE: POl.ISHED SILICON BU>CK SUK.'A(:t>

Fig. 7: Atomic force micrographs of the polished silicon surface that was ground with ductikstreaks gave a surface finish of R.a ~ 17.755 nm and R""", = 122.74 nm

Although the 25 mm depth of cut gives relatively higher Ra values, ductile streaks areevident particularly at the 35 mm/min feed. Getting partial ductile streaks on theground surface is a much better deal than a good surface finish as the former frompolishing experience (Fig. 8) (Venkatesh and Zhong 1994) shows that saturation hastaken place with the latter.

Generally on BK7 glass, surface roughness improved with decrease of feed rate asshown in Fig. 9. As can be seen from the graph, depth of cut of 25 mm provided thelowest Ra values at all levels of feed rate. Here again, it is seen that surfaces with ductilestreaks obtained at a higher feed rate (Figs. lOa and b) with higher surface roughnessare better than the surface with the better surface finish as the latter has reachedsaturation condition (Venkatesh and Zhong 1995). Displacement of material in the form



Polishing pOINd r: Ce02Grit sizo; 3 lAm

...........

.....-.......#tt:2#3

.........

min2

o

1Polh.hlng tim

4•••.....•..•....-....

••-,••••••••••

..e.".••".

o0.0

0.4

1.2

1.6

E:t

0.6

Fig. 8: Roughness value versus polishing time. Curve #1 with the largest amount of ductilestreaks and maximum roughness achieved better surface finish faster than curve #3

which has the best surface finish (Venkatesh and Zhong 1995)

Surface Roughness vs Feed rate(Grit D126-BK7 Glass)

0.7

0.6

0.5

E 0.42-~ 0.3

0.2

0.1

0.408 -+- doc=5um

...-doc=15um

-+- doc=25um

0+-------.------,---------,5 20

Feed rate (mmlmin)

35

Fig. 9: Surface roughness obtained lJy plano grinding at difJerent feedsand depth of cuts on BK7 glass

of chip on the ground surface in Fig. lOb shows evidence of plastic flow due to lateralploughing action when critical depth of cut is reached. A similar chip has been observedat a higher feed 50 mm on a precision LOH aspheric generator (Venkatesh 2003).

PertanikaJ. Sci. & Techno\. Supplement Vol. 12 No.2, 2004 121


(a)

(b)

Fig. 10: SEM micrographs of ground BK7 glass surfaces are shown at depth of cut of 25 mm and feed of(a) 5 and (b) 2Omm/min. Formation of a chip due to lateral ploughing action can be seen in (b).

Pyrex glass which has higher softening temperature (1093°K) and lower linearexpansion coefficient (3.25xlO-6/K) than BK7 yielded transparent surfaces at a lowerdepth of cut of 10 mm at feeds of 2.5 mm/min with a Ra value of 89 mm (Fig. 11). Fig.11 (b) shows diamond ground surfaces with 3 circular tracks corresponding to theconical configuration of the diamond wheel shown in Fig. 11 (a).

This wheel is traditionally used for internal grinding and a slight conicity of thewheel end is to provide grinding relief. When used as a surface grinding wheel with avertical axis the conicity gets more pronounced. It has been observed that the adjacenttracks get polished more easily than the central track in Fig. 11 (b), showing that thecentral track has reached a saturation stage seen in polishing curves (Venkatesh andZhong 1995).

Using the wheel geometry and ground surface structure, the critical feed and depthof cut for partial ductile mode grinding can be determined. As clearly shown in Fig.

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II

I _----------;-16 ---t------::':--....,,....:.::..;-r;:~-~------_Ja ........

B' "I;r/--

(a)

(b)

~ 2mm ~

0' C'" 1.35mm --,B'

y

}\ (c)Fig. 11: (a) shows exaggerated conicity oj grinding wheel (actual cone angle wiU be about 179.4').

(b) Plan view oj ground surface whose diameter is less than wheel diameter, because depth oj cutis less than y. The central part despite being fractured is transparent and the outer part

is translucent despite the partial ductile streaks. (c) Enlargement oj triangulararea O'B'A in (a) Jor estimation oj critical depth oj cut (y?

11 (c), depth of cut is higher at the central than the shoulder region. Substantialimprovement in surface quality of the shoulder region indicates the existence of thecritical depth (y'). Using the triangular cross section of the tip of grinding wheel, thecritical depth of cut is about 7 mm where partial ductile mode grinding initiates andwhere fracture areas end (Mon 2003).

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CONCLUSIONS

During aspheric generation the following observations are made:1. Resinoid-bonded diamond wheel gave better results for both Si and Ge than metal

bonded wheels.2. Ge which is a much more expensive material than Si is easier to grind and polish.

Germanium gives massive amounts of ductile mode streaks.

During high speed plano grinding, results are as follows:1. Ductile mode grinding streaks occur at higher depth of cut with BK7 optical glass

whereas with Pyrex glass they occur at a lower depth of cut when a minimum feedis used in both cases.

2. In the case of Pyrex glass very fine depth of cut and feed are essential for gettingtransparency over large areas, whereas in the case of BK7 such areas are scanty.

3. With silicon, considerable amount of ductile streaks are obtained at a higher depthof cut just like BK7 glass.

4. It has been observed with silicon, BK7 and Pyrex glass that larger amount of ductilestreaks is preferable to a ground surface with better finish in order to get anexcellent polish surface finish. This is due to saturation effect observed in polishingcurves.

ACKNOWLEDGEMENTS

Sincere thanks are due to the Ministry of Science, Technology, and Environment,Malaysia for the IRPA Funding on Industrial Diamonds, Vote 72255 and to INTELMalaysia, Penang for their research grant on Milling of Silicon Dies, Vote 68837.

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FANG, F. Z. and L. J. CHEN. 2000. Ultra-precision cutting for ZKN7 glass. CIRP Ann. 49(1): 17-20.

FANG, F. Z. and V. C. VENKATESH. 1998. Diamond cutting of silicon with nanometric finish. CIRP Ann47(1): 45-49.

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HUNG, . P. and Y. Q. Fu. 2000. Effect of crystalline orientation in the ductile-regime machiningof silicon. Int. J A. Manu! Eng. 16: 871-876.

KONIG, W. and V. SINHOFF. 1992. Ductile grinding of ultra-precise aspherical optical lenses. Int. Sym.optical Systems Design, Berlin.

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TONSHOFF, H. K., R EGGER, W. LoNGERlCH and D. PREISING. 1998. Superfinishing ceramics. Manu!Eng. (Feb): 52~0.



VAN LIGTEN, R. F. and V. C. VENKATESH. 1985. Diamond grinding of aspheric surfaces on a CNC 4Axis machining centre. CIRP.Ann. 34(1): 285-298.

V~'1 UGTEN, R. F. and V. C. VENKATESH. 1986. Invention - mmachine to make aspheric surfaces.Patent Disclosure: ational University of Singapore.

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VENKATESH, V. C. and Z. ZHONG. 1995. Semi-<luctile grinding and polishing of ophthalmic asphericsand spherics. CIRP Ann. 44(1): 339-342.

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Precision Grinding of Hard and Brittle Materials

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